This invention relates to a system for injecting a bolus of liquid into a patient to carry out a measurement of cardiac output by thermodilution and, more particularly, to a system including a filter to prevent the introduction of bacteria or particulates along with the injectate into the patient.
One of the common procedures during operations is the monitoring of cardiac output. Although some systems have been proposed to carry out noninvasive cardiac output, by far the most common method is to utilize thermodilution. The method is invasive and utilizes a catheter, such as a Swan Ganz catheter, that is positioned within the heart. A liquid having a known temperature, normally chilled, is introduced through the catheter into the right atrium and a temperature sensor, positioned in the pulmonary artery senses the change in temperature of the blood exiting the heart. By a correlation of time and temperature drop, a measurement of cardiac output can be readily calculated.
In common practice, the injection of the cold bolus of liquid is carried out manually by attending personnel through use of a syringe that has been chilled to the appropriate temperature and, in recent times, the injection may be automatically carried out by a pump means of certain types.
In either case, whether the injection of liquid is achieved by a manually operated syringe, or by an automated pump, a problem exists in insuring that the injectate itself does not introduce bacteria or other harmful substances into the patient. This is particularly important in that the injectate itself is introduced directly into the heart and therefore is carried rapidly through the patient's circulatory system.
In typical thermodilution applications, multiple injections are carried out on the patient over a fairly extended period of time. Generally, the same set up may be used, for example, for 72 hours and each reading of cardiac output may require 3-5 injections to insure the integrity of the cardiac output results.
Accordingly, there is an opportunity for bacteria or other contaminants to enter the system and thrive in the moist environment and eventually find its way into the injectate liquid that is delivered to the patient. One means could be through the various connectors in the tubing, stopcocks, leakage from the continuous use or the phenonema known as blow by. In that phenomena, liquid injectate is forced past the normal syringe seal when high flows are used, typical of thermodilution in the range of 150-220 cc./min, and the seal between the syringe plunger and inside of the syringe barrel cannot take the momentary high pressure. Thus some liquid leaks past the seal outside the closed environment within the syringe. That liquid may remain on the outside of the seal within the syringe barrel and later be reintroduced past the seal and into the injectate or later uses of the syringe. Also, since the external part of the syringe plunger is outside the closed environment, it is touched by personnel and contamination can thus directly infect the plunger and migrate to the interior of the syringe.
Such blow by thus could allow bacteria to form in the liquid during the time outside the closed environment of the syringe and thus it is a possible source of contamination.
In addition, of course, the syringe and/or other components may become unsterile by openings in the sterile packaging during shipping, handling or the like.
A study of the problems of introduction of bacteria or other contaminants into the thermodilution injectate is reviewed in an article in Heart and Lung, March 1988, Vol. 17, No 2 pgs 121-128 entitled "Sterility and Efficiency of Two Methods of Cardiac Output Determination: Closed Loop and Capped Syringe Methods".
One solution currently proposed to lessen the risk of bacteria contamination is described in Riuli U.S. Pat. No. 4,713,060 where a sheath is provided enclosing the otherwise open portion of a syringe to prevent foreign contamination from entering the syringe. Again, however, the problem of blow by is not completely solved in that the liquid that leaks past the syringe seal may remain within the sheath and provide an environment for bacteria growth. That liquid can later be reintroduced back through the syringe seal and contaminate the injectate that is directed to the patient.